If you found yourself locked up against your will in a psychiatric ward, you would probably do your best to get out. But in 1969 a group of people did just the opposite — they tried to get in. A young American psychologist called David Rosenhan persuaded seven friends (two psychologists, a psychiatrist, a doctor, a housewife, a painter and a student) to see whether they could convince doctors that they were mentally ill simply by claiming to hear voices. Now previously unpublished notes from Rosenhan’s private archive reveal what the experience was really like.

Between 1969 and 1972 the team of “pseudo-patients” presented themselves at 12 different US hospitals in five states on the East and West coasts. What would a sane person have to do to convince a doctor they were insane? Not a lot, it seems.

Having claimed to hear words from “thud” and “empty” to “hollow”, words selected because they had never been recorded in psychiatric literature, every pseudo-patient was admitted to hospital for varying lengths of time, from 7 to 52 days. They were given diagnoses of schizophrenia and prescribed a total of 2,100 pills (only two of which were swallowed; in preparation for the study the pseudo-patients had learnt to “cheek” any medication).

Other than giving false names and inventing voices, the patients were to answer all other questions honestly. If they were admitted to hospital they were to say that they felt better and that the voices had disappeared. Not one member of staff suspected them of faking it.

Not only could the doctors be fooled, but once Rosenhan had been given a diagnosis the hospital staff began to read into his actions. His study required him to keep detailed observations, so the staff often saw him writing notes. This was described as “writing behaviour”. When one of the other pseudo-patients, a professional painter, sat down one day and began to paint, this was deemed “painting behaviour”.

Now aged 79 and barely able to speak after a stroke, Rosenhan, who lives in a nursing home in Palo Alto, California, allows me to look through the boxes containing his archive of papers. His unpublished notes describe his own repeated trips to psychiatric hospitals using the pseudonym David Lurie. He found the experience shocking, not because he was able to trick the doctors into admitting him, but because of the way he was treated the moment he had been labelled mentally ill. “I can still recall my own impulse to go up to the nurses and say, ‘You think I’m David Lurie, well I’m not, I’m David Rosenhan, professor of psychology. It was only my anticipation of their likely response — ‘Do you often think you’re a professor of psychology?’ — that stopped me doing it.”

Looking through Rosenhan’s notes, it’s clear that the whole experience has had a lasting effect on him. “Months spent as a pseudo-patient have evoked in me passions that I hardly believed I knew existed,” he says. He found himself in a Catch-22 situation: even when he told the doctors that he felt better, he still wasn’t allowed to leave. “The only way out was to point out that they were correct. They said I had been insane, I was insane, but I was getting better. It was an affirmation of their views.”

Even the pseudo-patients whose work meant that they were familiar with psychiatric hospitals found their experiences as a patient unsettling. Some carried out the experiment many times, becoming more nervous with each new hospital they visited. They also found the staff reluctant to leave the nurses’ station known as the “cage”, and estimated that the time spent talking to patients to be a mere six and a half minutes a day.

The publication of Rosenhan’s paper On Being Sane in Insane Places in the eminent journal Science in 1973 assured his place in the history of psychology. The timing of the study could not have been better, following criticisms of the practice of psychiatry by the Scottish psychiatrist R. D. Laing and influential books on the subject by Thomas Szasz and Erving Goffman.

The profession reacted furiously, complaining that the fact that they could be tricked did not undermine their methods of diagnosis. It was not their job, they said, to look for hoaxers. Patients could present with fake symptoms in any field of medicine and be prescribed unnecessary treatment. Doctors rely on patients to tell the truth and do not expect them to invent symptoms. After all, a person who goes to his doctor complaining of severe stomach pains would be taken at his word and possibly even admitted to hospital.

But Rosenhan argued that however much psychiatry might want to be viewed like any other branch of medicine, the difference was the lack of further tests to confirm a diagnosis. None of the decisions to diagnose schizophrenia in the pseudopatients was reversed, even for the patient who had been observed for 52 days. Rosenhan wondered how a doctor who could not even tell which patients had mental health problems could ever expect to distinguish between different types of mental illness.

Rosenhan’s friend and colleague, the Stanford University psychologist Lee Ross, believes that he relished the controversy: “I don’t think he minded being attacked. There would be no point in doing the study if he didn’t think he’d get the result he got.” After the study one hospital challenged Rosenhan to send more pseudo-patients, insisting that doctors would be able to spot the fakers if only they knew to look for them. For the next three months they monitored their admissions and uncovered 41 hoaxers. Mischievous as ever, Rosenhan had sent none.

The study demonstrates something much broader about human nature, something that psychological research has confirmed many times: once we have a view about a person, we look for anything that backs up our ideas and we explain away any evidence to the contrary. Ellen Langer, a professor of psychology at Harvard University, says that within the health sphere this can have far-reaching consequences, “When a patient walks in to see a therapist that person is seen through a patient lens. Very ordinary behaviours are now going to seem extraordinary or pathological.”

Many believe that this is still the case today. Rufus May is a British clinical psychologist who bridges the divide between service user and health professional, having spent some time in a psychiatric hospital after a diagnosis of schizophrenia at the age of 18. “Staff do see patients in a compassionate way, but they also view them with fear because they see them as diagnoses,” he says. “That’s still very clearly the case. There are lots of good people working in the system, but the role often confines people.”

So would psychiatrists still be fooled if the pseudo-patient study was repeated today? Ethics committees would be unlikely to allow academic psychologists to repeat the study, but there have been attempts by journalists to do something similar. As most mental healthcare now takes place in the community, it would be hard to get admitted to hospital just for claiming that you can hear voices.

Methods of diagnosis have improved — indeed, there is now a movement against labelling people with a diagnosis at all. Some clinicians today prefer to describe and treat symptoms in place of giving a formal diagnosis. While the psychiatric profession did take notice of the pseudo-patient study, it is sad that it required healthy people to live through the experience before anyone listened.

You can hear more about the study in Claudia Hammond’s programme Mind Changers today at 11am on BBC Radio 4 . You can also listen to it on playback